Iron refining process



.to produce steel. 'is considerably more complex than would appear from United States Patent 3,030,202 IRON REFENING PROQESS Zsigmond de Galocsy, 125 Ave. Louise, Brussels, Belgium N0 Drawing. Filed Apr. 7, 1960, Ser. No. 20,536 Claims priority, application France Apr. 9, 1959 8 (Ilaims. (6!. 75-51) This invention relates to the refining of iron.

The refining of iron broadly designates the burning-up of carbon and other elements present in the iron as cast However, the actual process involved the face of the above simplified definition. In the iron bath, the carbon contained within the iron does not burn into carbon dioxide, but instead is gasified into carbon monoxide. It is only later, as this carbon monoxide bubbles out of the bath and contacts the free oxygen in the overlying atmosphere that it undergoes an additional combustion to generate carbon dioxide. This subsequent combustion however clearly has no bearing on the actual metallurgical process. The truth of the above statement is not altered by the fact that in a converter the oxygen first forms carbon dioxide with the carbon since this carbon dioxide thus formed is reduced to carbon monoxide. The dioxide acts so to speak as an oxygen car-rier containing the oxygen in combined state.

In an openhearth furnace, an oxygen-carrier is similarly initially formed in the form of metal oxides, which then act to gasify the carbon to produce monoxide by the action of the combined oxygen carried by the oxides. Here again the carbon monoxide is formed indirectly. it is a surprising fact that no attempts have heretofore been made in the iron and steel-making arts to induce a direct gasification of the carbon although it is apparent that considerable advantages would be gained from such a step. This is especially surprising since the use of oxygen nozzles for other purposes is known in various iron processes as in the openhearth process, the so-called LD process, rotor, Kaldo and Irsid processes.

It is therefore a basic object of the present invention to provide an improved iron refining process involving a direct gasification of the carbon and other constituents to carbon monoxide in the iron bath.

My invention is based on a concept that can best be described as the maturity of the carbon and oxygen ingredients of the refining reaction, with regard to gasification. That is, the said ingredients can be described as mature if they are in a condition such that they will react instantaneously to produce carbon monoxide.

The degree of maturity depends primarily on the temperature of the reagents. Hence, the desired maturity enabling the direct gasification reaction to take place in accordance with the invention, will generally be fulfilled only if both reagents (carbon and oxygen) are, before being contacted, brought to a temperature higher than the upper boundary of the Boudouard diagram.

Thus, it would by no means be sufficient, in a converter for example, if only the iron, or only the carbon contained in it, were brought within the desired temperature range indeed; this is the case in all conventional blast-refining processes. However, the air or oxygen (and any oxygen carriers) are always introduced cool, that is in a condition in which it is immature as regards gasification. This results in the above mentioned initial formation of carbon dioxide, i.e. the gasification into carbon monoxide is delayed. On the other hand the carbon dioxide thus formed is mature for gasification, since it has been brought to this condition within the bath itself by the strongly exothermal character of the reaction, and is therefore able to revert to carbon monoxide by an endothermal reaction process. The initial formation of carbon dioxide is further facilitated by the fact that the 3,936,202 Patented Apr. 17, 1962 carbon here involved in the reaction is not free carbon but is combined with iron. Owing to this the equilibrium position in Boudouards diagram is further displaced towards still higher temperatures.

The serious drawbacks that flow from such primary formation of carbon dioxide, in a converter (whether provided with a bottom blast or with an oxygen torch blast) may be indicated as follows:

in the first place, at the point where the oxygen first contacts the bath, an excessively high temperature is generated, and this, in the case of a bottom-blast converter, leads to a premature destruction of the converter bottom, while in the case of surface blast if promotes a surface evaporation or sublimation of the iron. This is true both in converters and in hearth (e.g. openhearth) furnaces. The well-known, objectionable red vapor is formed in this way.

Secondly, in a bottom-blast converter, an excessive amount of iron is initially burned. This is also a consequence of immaturity of the constituents. It can easily be demonstrated by a computation based on the equilibrium constants that in the case of media that are mature for gasification, the formation of FeO is inhibited.

Thirdly, the gasification process is retarded owing to the indirect'manner in which the carbon monoxide is formed.

Tests have been made previously to this invention, in a bottom-blast converter, for refining the iron with a cool mixture of pure oxygen and water vapor or carbon dioxide. The gasiiication was then found to proceed on the following lines.

In an initial stage, only the free oxygen reacts and forms carbon dioxide. Part of the heat thus generated is taken up in heating the injected oxygen carriers to their point of gasification-maturity. Only then is the carbon able to reduce the carriers. In this case, also, there is delayed gasification. In contrast with the prior teaching that an endothermal reaction tends to cool the bottom of the converter, it has been found that owing to the displacement -of the reduction area towards a higher elevation in the converter, the converter bottom is destroyed at an even faster rate than with the more conventional procedure, thus confirming the statements made above. Moreover, due to the fact that the free oxygen is reduced first, only a small proportion of carbon is available for reduction of the oxygen carrier.

An entirely different situation obtains when one applies the concept of gasification maturity, that is, when both the carbon and the free and combined oxygen are treated outside the converter to bring them to a point of gasification-maturity. In this case, the free oxygen enters directly, and with much less generated heat, into reaction with the carbon in the iron, to produce carbon monoxide, while the oxygen carriers also react without any delay. Near the bottom of the converter a more uniform temperature is maintained. In addition to the shortened time of treatment of the charges thus made possible, the following further advantages are had:

The free oxygen is no longer in a condition of priority over the combined oxygen for the reaction with the carbon. The exothermal and endothermal reactions proceed concurrently without interfering with each other. The carbon fraction that is able to react with the oxygen carrier is accordingly increased. in other words a saving in oxygen is obtained.

The gasifioation-mature mix brings with it a greater amount of heat, i.e. more than the heat taken up in the reduction of the oxygen carriers. Consequently, additional cooling agents, such as scrap, ore, limestone, lime, and the like, can now be added in greater proportion into the converter than was heretofore possible.

Mathematical analysis shows that in the case, for example, where a bottom-blast converter is supplied with a mixture made up of ordinary air and water vapor at not higher than about 1000 C., the nitrogen concentration in the exhaust gases is reduced to less than about 50%, as against about 73% for cool-air operation. A similar nitrogen concentration could not otherwise be attained except by raising the oxygen content in the cool air to about 45%. The excess heat would at the same time make it possible to melt about 140 kg. added scrap per metric ton of raw steel.

The nitrogen content would, moreover, drop to about 35% if the blast mixture used, at 1000 C., contained only about 30% oxygen and steam. In the first instance, the need for oxygen has been entirely eliminated, while in the second instance the amount of oxygen required would be reduced by half. It is thus seen that in order to produce low-nitrogen steel in a converter, it becomes possible to do without oxygen-enriched blast entirely.

In the surface-blasting refining processes, oxygen cmriers are not generally used since in that case the excess heat can be used to melt added scrap. However, if the inventive teaching relating to gasification-maturity is applied to such processes, it would become possible to add at least the same amount of scrap and save a considerable amount of oxygen while preventnig any surface evaporation or sublimation.

Tests conducted with open hearth furnaces, using a cool blast mixture comprising oxygen and steam, have not been successful the steam being unable to react with the carbon.

Thus according to the invention in a refining process, the blast mixture should be brought to its point of gasiflcation ion-maturity (1400 C. to 2000" C.). The blast mixture comprises air or oxygen of suitable concentration, and an oxygen carrier. As the oxygen carrier steam and carbonic dioxide may be used. Also, solid carriers may be used such as ores, limestone, iron and manganese, the hydroxides and carbonates thereof and the like. The simultaneous use of solid oxygen carriers is especially recommended where the refining medium is discharged on the surface of the bath.

In such cases, it is especially advantageous to carry the refining medium (gasification reagents) to their point of gasification-maturity by using the hot exhaust gases from a burner. According to this method, a fuel having a very low nitrogen and sulfur-concentration, such as charcoal dust, sawdust, coking gas, natural gas, liquefied oil gases, oils, and the like, is completely burned in a burner together with air that may or may not have been pre-heated, or oxygen of the desired concentration. Further, water vapor and/or carbon dioxide and/or solid oxygen carrier substances, are added to the combustion gases. The gases are then passed over or into the bath while at a temperature of about 1500 C. This device may be called a flamethrower. The mixture of hightemperature combustion gases and solid oxygen carriers is discharged over or into the bath with a high kinetic energy, such as a velocity of at least 100 meters per second.

By a suitable choice of the fuel, oxygen concentration, character and amount of oxygen carriers, and temperature of the blast, the conditions may be easily and rapidly controlled to obtain optimum gasification maturity at all times. In some cases a reducing flame may be used.

The carbonates and hydroxides undergo decomposition already within the burner, and their carbon dioxide and water vapor content is aded to that of the combustion gases. The roasting of any limestone or similar constituents present thus occurs within the flame itself, and no heat is taken out of the bath for the purpose. Another advantage is that the solid oxygen carriers which have reached their point of maturity, and are possibly in molten form, and the lime if any, also react at an extremely rapid rate.

The above-described flame-thrower technique may be used both in a surfaceor bottom-blast converter process and in a hearth-furnace process. An especially satisfactory combination is obtained in the base of a bottom-blast converter process. Thus, part of the refining reaction may be performed, as before, 'by the bottom-blast, or preferably by a blast mixture that has been brought to its maturity point, while the remaining part may be effected by the flame-thrower. Depending on the particular iron charge used and grade of steel desired, either one or the other, or both simultaneously of the two methods may be used. This procedure makes it possible to produce, in a bottom-blast converter, 21 grade of steel having a very low nitrogen content even if cool air is discharged as the bottom blast. It also is possible, in the case of at Thomas process, to adjust the rates of removal of the phosphorus and the carbon in the most favorable proportion desired.

According to the invention, the flame-thrower process may be performed in either of two chief Ways. The combustion may be carried out in a burner so constructed that the flame is formed within the burner; or the burner may be arranged so that the combustion will occur at the outlet. In this case the flame may, according to the invention, be made to expand within the free space of the furnace at a selected distance from the bath surface, or it may be immersed within the bath. I

In one specific embodiment, one or more burners may be fixedly installed in the sides of the converter or hearth furnace, and directed to discharge at an angle on to the bath.

Depending on the conditions used, the flame thrower may be internally lined with refractory material and provided with cooling means, or may be constructed as a simple tube if this is possible.

Any suitable type of heat exchange apparatus may be used in order to heat the refining medium to its point of maturity, and/or for the preliminary heating of the ingredients fed to the burner. The type of heat exchanger operating with moving heat carriers is especially advantageous. Instead, electrical heating apparatus may be used if desired.

However, it may be especially desirable in order to eliminate any external energy sources, to use the latent and sensible heat contents of the converter exhaust gases. One convenient way of doing this is to use a rotary or rocking refining furnace. Where the weight of the charge is relatively heavy, and would lead to excessively great peripheral velocities for the rotating furnace, according to the invention axial drives elements may be mounted within the rotary furnace along side the walls thereof, which serve during rotation to drive the molten metal bath upwards and allow it to fall back in a spray from a substantial elevation. This further accelerates the process and simultaneously removes undesirable constituents into the slag, at a time when the carbon monoxide bubbles are no longer capable of maintaining agitation in the bath. The driver or guide elements may desirably be made so as to follow a helical path.

It will be noted that the method of the invention is based upon a simultaneous control of two temperatures, the reaction temperature proper which is the temperature of the refining reagents within the bath, and the combustion temperature of the flame-thrower where this is used. In most refining processes the combustion temperature should not generally exceed about 1500 C. On the other hand, it should be adjusted to more than 2000" C. in cases where scrap is to be melted. At all events the reaction temperature, in a refining process, should be maintained below about 1800 C. The two temperatures involved are controlled by adjusting the ratio of free to combined oxygen.

What I claim is:

1. In an iron refining process, the step of discharging into molten iron a mixture of oxygen-containing con- -ndh.

stituents selected from the group consisting of air and oxygen, and oxygen carriers selected from the group consisting of steam, carbon dioxide, and powdered oxygencontaining ores, limestone and the hydroxides and carbonates of iron and manganese, said mixture being at a temperature within the range of approximately 1400 C. to 2000 C. and creating an oxygenating atmosphere.

2. An iron refining process as in claim 1; wherein said mixture is discharged into the molten iron in the form of a jet having a velocity of at least 100 meters per second.

3. An iron refining process as in claim 2; wherein said jet is directed against the surface of the molten iron.

4. An iron refining process as in claim 2; wherein said jet is directed into the molten iron below the surface of the latter.

5. In a converter steel-making process, the steps of maintaining a bath of molten iron, separately burning a fuel to produce at least one jet of combustion gases at a temperature between approximately 1400" C. and 2000 C. and a velocity of at least 100 meters per second, adding free oxygen and combined oxygen containing oon stituents to said jet, said combined oxygen containing constituents being selected from the group consisting of steam, carbon dioxide, powdered oxygen-containing ores, limestone and the hydroxides and carbonates of iron and manganese, and discharging said jet into said bath of molten iron so that an oxygenating atmosphere is created and the carbon in said bath is directly gasified to carbon monoxide for converting the iron to steel.

6. In a converter steel-making process, the steps as in 6 claim 5; wherein said jet is discharged into said bath at a substantial depth below the surface of the bath.

7. In a converter steel-making process, the steps as in claim 5; and wherein the ratio of free oxygen to combined oxygen containing constituents added to said jet is controlled so as to maintain a reaction temperature within said bath lower than 1800" C.

8. In a converter steel-making process, the steps as in claim 7; further comprising adding scrap to the iron in said bath, and controlling the ratio of free oxygen to combined oxygen containing constituents added to said jet so as to maintain a combustion temperature in said jet of at least approximately 2000 C.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Sweetser: Blast Furnace Practice, 1st edition, 1938, McGraw-Hill Book Co. Inc., New York, pp. 167-169. 

5. IN A CONVERTER STEEL-MAKING PROCESS, THE STEPS OF MAINTAINING A BATH OF MOLTEN IRON, SEPARATELY BURNING A FUEL TO PRODUCE AT LEAST ONE JET OF COMBUSTION GASES AT A TEMERATURE BETWEEN APPROXIMATELY 1400* C. AND 2000* C. AND A VELOCITY OF AT LEAST 100 METERS PER SECOND, ADDING FREE OXYGEN AND COMBINED OXYGEN CONTAINING CONSTITUENTS TO SAID JET, SAID COMBINED OXYGEN CONTAINING CONSTITUENTS BEING SELECTED FROM THE GROUP CONSISTING OF STEAM, CARBON OXIDE, POWDERED OXYGEN-CONTAINING ORES, LIMESTONE AND THE HYDROXIDES AND CARBONATES OF IRON AND MANGANESE, AND DISCHARGING SAID JET INTO SAID BATH OF MOLTEN IRON SO THAT AN OXYGENATING ATMOSPHERE IS CREATED AND THE CARBON IN SAID BATH IS DIRECTLY GASIFIED TO CARBON MONOXIDE FOR CONVERTING THE IRON TO STEEL. 